1. Field of the Invention
The present invention relates generally to the field of systems for weaning a patient from a mechanical ventilator. More specifically, the present invention discloses a method and apparatus for augmenting the flow of oxygen into the patient's lungs during the weaning process by inserting a transtracheal catheter through the tracheostomy tube.
2. Statement of the Problem Patients with acute lung injury or respiratory failure are often treated by use of a mechanical respirator or ventilator. One of the most common mechanical respiratory units employs a tracheostomy tube inserted into the patient's trachea to force oxygen under pressure into the lungs. The distal portion of the tracheostomy tube includes an inflatable, occluding balloon which functions as a cuff in the throat to minimize retrograde air leaks during the inflation cycle of the ventilator. The ventilator periodically increases and decreases the pressure within the patient's airway to deliver oxygen to, and remove carbon dioxide from the lungs. Virtually the entire flow occurs through the tracheostomy tube.
It is also well known that mechanical ventilation techniques produce detrimental side effects. In particular, the elevated airway pressures during mechanical ventilation can further damage the patient's lungs and greatly increase the risk of barotrauma (for example, subcutaneous emphysema, pneumothorax, and pneumopericardium) and pulmonary interstitial and alveolar edema. Efforts to limit the airway pressure during mechanical ventilation can sometimes result in inadequate blood gas levels. Pressure limits can also result in permissive hypercapnia, in which carbon dioxide levels are allowed to increase. This can cause the pH of the patient's blood to become dangerously acidotic. The patient tends to stay in respiratory failure and the patient's renal, hepatic and cerebral functions can also be impaired.
In addition to damaging the patient's respiratory tree, mechanical ventilation also tends to negate the natural augmentation of venous return that normally attends spontaneous respiration, and decreases cardiovascular output. These factors often create a progressive, downward spiral for patients that have been placed on mechanical ventilation.
As a result of these detrimental side effects, health care professionals attempt to limit the duration of mechanical ventilation to a minimum and to wean patients from mechanical ventilation as soon as possible. The conventional approach for weaning ventilator patients involves periodically removing the patient from the ventilator for increasingly long periods of unassisted breathing. However, an abrupt transition from mechanical ventilation to unassisted breathing is sometimes too difficult for the patient and can result in episodes of acute respiratory distress.
In contrast to the "closed" ventilators discussed above, a number of open oxygen delivery systems have been invented in the past. Open systems deliver oxygen into the nostril, mouth, or the trachea while keeping the patient's lungs open to the atmosphere. The patient is allowed to continue breathing spontaneously in such open systems. These systems are much more comfortable to the patient than closed systems. For example, the patient is usually permitted to speak, eat, and drink freely. Open delivery systems are typically designed for long-term use by patients who are capable of self-breathing.
One example of an open delivery system is the "SCOOP" transtracheal catheter manufactured by Transtracheal Systems, Inc. of Englewood, Colo., and described in U.S. Pat. Nos. 5,090,408 and 5,181,509. One system for continuous flow augmentation and ventilation of a patient using a transtracheal catheter is discussed in the applicant's U.S. Pat. No. 5,101,820, issued on Apr. 7, 1992. The "SCOOP" transtracheal catheter is made of a bio-polymer of 70-90 Shore A durometer hardness that resists kinking and crushing. The internal tubing is radio-opaque. The total length is approximately 20 cm. The internal length (from the neck flange to the distal end of the catheter) is approximately 9 cm to 13 cm. The inside diameter of the tubing is 1.7 to 3.0 mm and the outside diameter is 3.5 mm or less. Oxygen flow rates of up to six liters per minute are possible without exceeding the two psi maximum back pressure of conventional delivery systems.
Nahum et al. have suggested combining a conventional closed mechanical ventilation system with tracheal gas insufflation ("TGI") to augment alveolar ventilation without distention or other forms of alveolar damage. Nahum et al., "Tracheal Gas Insufflation During Pressure-Control Ventilation", American Review of Respiratory Diseases (vol. 146, pages 1411-1418, 1992). Constant-flow ventilation (CFV) catheters are positioned in the main bronchi or in the trachea to augment air flow to the patient's lungs without substantially increasing the airway pressure. However, Nahum et al. propose use of TGI in combination with, or as an adjunct to conventional mechanical ventilation systems, and not a means for weaning the patient from mechanical ventilation.